This invention relates to analytical electron microscopy and more particularly to an analytical method and associated apparatus involving an analytical electron microscope for determining the composition of a sample containing two or more elements.
Previous use of analytical electron microscopes to determine the elemental composition of samples has involved the use of measured x-ray intensities in a technique identified as x-ray energy dispersive spectroscopy. The fundamental difficulty in applying this technique is the determination of the proportionality constant relating sample composition to measured x-ray intensity. As background, the number of x-ray photons detected per incident electron in a pure element sample is given by ##EQU2## where I is the number of detected x-ray photons, .phi. is the total electron dose during the x-ray detector live time, Q is the ionization cross section, .omega. is the fluorescence yield, .alpha. is the x-ray partition function, .epsilon. is the detector efficiency, .OMEGA./4.pi. is the fractional solid angle subtended by the x-ray detector, A is the sample atomic weight, .rho. is the sample density, N.sub.o is Avagadro's number, and t is the sample thickness. The elemental composition of a two element sample can be determined using the ratio method. ##EQU3## where K.sub.AB is a "K" factor equal to ##EQU4## and C.sub.A and C.sub.B are concentrations of elements A and B, respectively, in weight percent. The above calculations are applied for all components with the remaining relationship being that the total concentration equals 100%. In many instances, the factor K.sub.AB has been previously determined by experimental measurements on well characterized multi-element standards when conditions permit the preparation of the standard with known concentrations. However, the multi-element standard technique has some problems in that it may be difficult to prepare the multi-element standard particularly where the elements do not readily form alloys.
Accordingly, one object of the invention is a method of determining the concentration of elements in a sample by analytical electron microscopy. Another object of the invention is a method which does not require the preparation and use of multielement standards. These and other objects will become apparent from the following detailed description.